Alkali metal cathode lamps

ABSTRACT

The radiation emitting cathode of spectral source lamps often is a hollow cup, the interior of which contains a coating of the spectrally emitting element or elements. The forming of such a coating of an alloy of an alkali metal (or metals) with, say, tin in the presence of some boron is proposed, resulting in higher melting points and lower vapor pressures, thereby allowing higher operating lamp currents and consequent spectral radiation intensity. The coating material is formed, say, directly on the interior of the cathode cup (say, of titanium) by fusing an alkali metal borohydride with tin, thereby avoiding the need to handle pure alkali metal. The hydrogen gas liberated during alloy formation removes some of the contaminants (e.g., oxides). A boron-containing, glassy slag may be readily separated from the alkali metal alloys. Specific examples in which the alkali metal component is sodium, potassium, or a mixture of sodium and potassium are disclosed. The other metal may be, for example, tin or lead.

Unite Sttes atet [ 72] inventors John W. Vollmer Norwalk; LaurencePellier, Westport, Conn. [21] Appl. No. 656,564 [22] Filed July 27, 1967[45] Patented Feb. 2, 1971 [73] Assignee The Perkin-Elmer CorporationNorwalk, Conn. a corporation of New York [54] ALKALI METAh CATHODE LAMPS5 Claims, 1 Drawing Fig. [52] US. Cl 313/218, 313/217,313/311, 313/339[51] lnt.Cl H0lj 17/04 [50] Field ofSeai-ch 313/314, 315, 317, 318, 217,348, 350, 271,244, 326, 339, 346, 355, 218, 328, 311

[5.6] References Cited UNITED STATES PATENTS 3,201,639 8/1965 Levi313/346 3,286,119 11/1966 Sugawara et a1 313/346 3,346,750 10/1967Huberetal 3,388,275 6/1968 Bettenhausen et al Primary Examiner-John W.Huckert Assistant Examiner-Andrew J. James Art0rney-Edward R. Hyde, Jr.

ABSTRACT: The radiation emitting cathode of spectral source lamps oftenis a hollow cup, the interior of which contains a coating of thespectrally emitting element or elements. The forming of such a coatingof an alloy of an alkali metal (or metals) with, say, tin in thepresence of some boron is proposed, resulting in higher melting pointsand lower vapor pressures, thereby allowing higher operating lampcurrents and consequent spectral radiation intensity. The coatingmaterial is formed, say, directly on the interior of the cathode cup(say, of titanium) by fusing an alkali metal borohydride with tin,thereby avoiding the need to handle pure alkali metal. The hydrogen gasliberated during alloy formation removes some of the contaminants (e.g.,oxides). A boroncontaining, glassy slag may be readily separated fromthe al kali metal alloys. Specific examples in which the alkali metalcomponent is sodium, potassium, or a mixture of sodium and potassium aredisclosed. The other metal may be, for example, tin or lead.

ALKALI METAL CATI-IODE LAMPS This invention relates to improvements inhollow cathode lamps of the type used as sources of spectral radiation.More particularly the invention concerns the preparation of hollowcathodes for such lamps, in which the active material on the interior ofthe hollow cathode holder includes at least one alkali metal.

INTRODUCTION One type of source of spectral radiation (which is usefulin spectroscopic analysis, for example by means of an atomic absorptionspectrometer) is the hollow cathode lamp. In such lamps the cathode iscup shaped and includes as at least a substantial portion of itsinterior surface a material including the element or elements, havingthe spectral radiation characteristic desired. For those elements havingsuitable physical characteristics (such as melting point, vapor pressureand electrical characteristics), the spectral element may be, forexample, a coating on the interior of a hollow cathode holder of anothermetal. If the element for which the spectral radiation is desired has,for example, extremely low melting point or high vapor pressure at theoperating temperature of the lamp, other techniques must be utilized.One such technique is the formation of an alloy of the desired element(or elements) with other metals.

The alkali metals as a class have very low melting points and very highvapor pressures relative to the normal operating temperature of thecathode of the lamp (around 400 C). It has already been proposed to usebinary alloys of the alkali metals (for example, sodium and potassium)with for example, lead, and utilize the resulting alloy (e.g., NaPb andKPb as the interior surface of a hollow cathode. Such prior techniquedoes not provide a complete solution to the problem in that theresulting binary (sodium-lead and potassiumJead) alloys still have relaively low (approximately 325 C. for NaPb melting points, therebynecessitating relatively low operating temperatures (and therefore bothlow current and radiation intensity). Additionally the formation of suchalloys in situations utilized in hollow cathode lamp production ispractically difficult because of the well known problems in handling theextremely chemically active alkali metals.

The present invention greatly facilitates manufacture, in that thematerials initially alloyed are both safer and require less expensiveapparatus in their handling, both prior to and during the alloyingprocess. Broadly the invention utilizes one or more alkali metalborohydrides and a suitable additional metal (for example, tin) to forman alloy containing the desired alkali metal, some boron, and theadditional metal. The resulting alloy also has a somewhat higher meltingpoint and lower vapor pressure at, say, 350 C. than the correspondingbinary alloys.

Accordingly, an object of the invention is the provision ofa simpler,more economical method of manufacture of a hollow cathode assembly foran alkali metal spectral radiation lamp.

Another object is the provision of a hollow cathode assembly for use ina spectral radiation lamp, utilizing an improved alkali metal alloy asthe emitting material.

Other objects, advantages and features of the invention will becomeobvious to one skilled in the art upon reading the fol lowing detaileddescription in conjunction with the accompanying drawing, in which:

The sole FIG. is a cross section through a hollow cathode assembly ofthe invention, including an interior coating of the alkali metal, boron,and additional metal alloy.

DESCRIPTION The drawing illustrates a finished hollow cathode assemblyin which a hollow cathode cup holder 22 (of, for example, pure titanium)has a coating 30 of an alkali metal, boron, and additional metal alloysubstantially covering the interior surface 24 of both the cylindricalsidewall portion 26 and the heavier bottom portion 27 of the holder.Such conventional hollow cathode holders are provided with a reducedportion 28 having a recess 29 for engagement with a pin (not shown) ofthe lamp in which they are used. which pin provides both mechanicalsupport for and the (negative) voltage connection to the cathodeassembly. The tertiary alkali metal alloy at 30 may either be prealloyedand then cast within the hollow cathode cup 22, or both the alloying andcasting may be done in the same holder 22 intended to be utilized in thefinished assembly, as will appear hereinafter.

A general description of how the alkali metal alloy coatings accordingto the invention may be made is given, followed by three specificexamples, utilizing different alkali metals. In the immediatelyfollowing description the alkali metal will be assumed to be potassiummerely for simplicity of expression; as will be seen not only maydifferent alkali metals be used, but even mixtures of different alkalimetals.

A small quantity of the alkali metal borohydride (e.g., KBI-L) ispositioned at the bottom of a hollow cathode cup (having its reduced end28 lowermost) or a suitable crucible of similar shape (which cruciblemay be made for example of graphite). The alkali metal borohydrides (inparticular, potassium and sodium borohydrides) are substantiallycompletely stable in dry air at room temperatures (these particularborohydrides have been maintained in (dry) air filled vials for severalweeks without any noticeable deterioration through chemical reaction).For long periods of storage, or at elevated temperatures (as in thesucceeding manufacturing steps) the borohydrides should preferably bemaintained under an inert atmosphere (for example, argon or at least dryair). For short term periods (such as during weighing or other simplemanipulative steps), the borohydrides may be exposed to normal ambientair (especially of only moderate humidity) without any appreciabledecomposition occurring. A substantially larger (on the order of tentimes as much by weight) quantity of relatively pure tin is then placedon top of the alkali metal borohydride, and the entire assembly heatedto cause first melting of at least the tin. and then decomposition ofthe alkali metal borohydride in a controlled manner, thereby evolvinghydrogen. The inert atmosphere is preferably constantly changed so as toflush away the evolved hydrogen.

A convenient apparatus is an enclosed centrifuge having an externalinduction heater and having inlet and outlet connections for theconstantly flushing, say, argon gas. Constant moderate current issupplied to the induction heater until the tin melts (at 232 C.)completely. The current is then slowly raised until evolution of thehydrogen (indicating the decomposition of the borohydride) starts. Therate of decomposition should be controlled by (manual) adjustment ofheater current, or more simply by turning the heater current switch onand off, to avoid violent hydrogen release and the consequent loss ofmaterial over the upper edge of the container (e.g., 23 of theillustrated hollow cathode holder). During the entire heating operationthe container and its contents are preferably slowly spun by thecentrifuge to assist in mixing of the ingredients and escape of thehydrogen gas.

The passage of the hydrogen gas through the molten tin has the desirableeffect of reducing any tin oxide which may be present (because ofsurface oxidation of the tin in its original form). After the bubblingstops (indicating that all of the alkali metal borohydride has given upits hydrogen), the heating current is completely turned off, and thegraphite crucible, hollow cathode holder, or other container is cooled.A glassy coating or slag is formed substantially on the upper surface(e.g., at 32) of the alloy, such slag 34 including a substantialproportion of boron compounds. This slag may be readily removed from themetallic alloy (e.g., of potassium, boron, and tin).

Although it is possible to recast the alloy in the same hollow cathodeholder (when such is used as the container in the foregoing steps),preferably the alloy is recast into a clean new cathode cup (of titaniumfor the exemplary alloy) either at this stage or at a later stage oflamp assembly. The desired shape of the final alloy coating at 30 may beobtained either by centrifuging or by nutating (i.e., turning about a"wobbly" generally vertical axis) the cathode or the lamp into which ithas already been installed while the alloy is molten, and then coolingto solidify the coating.

SPECIFIC EXAMPLES Specific Approximate Example useful range Sodiumborohydride 0. 123 0. 070-0. 165 Tin 0. 877 0. 930-0. 835

As previously stated, the container with the sodium borohydride in thebottom and the tin thereover is placed within a flushing inert (e.g.,argon) atmosphere. Thereafter heat is supplied to first melt the tin andthen to decompose the sodium borohydride (at about 500 C.), moderatingthe rate of hydrogen evolution to avoid loss of material; thetemperature is finally raised slightly to insure a complete melting ofthe alloy. All of the heating steps are preferably done with the hollowcathode cup or other container being slowly spun by the centrifuge. Uponcooling, the previously noted glassy layer 34 tends to form as adiscontinuous dispersion or group of particles over the sodium, boron,tin alloy surface (i.e., 32). These particles may be readily removed bymechanical means (i.e., physical scraping), and the tertiary sodium,boron, tin alloy is then preferably recast in a titanium hollow cathodeholder (22) (which may be identical to container as used in the previoussteps).

For a total of one gram of initial ingredients and therefore 0.123 gramsof sodium borohydride), approximately 0.014 grams of hydrogen will bereleased. This much hydrogen would occupy approximately 150 cc. atstandard conditions (atmospheric pressure and C. or 273 K) and willoccupy somewhat less than 500 cc. at the elevated temperature(approximately 500 C. or 773 K) utilized during the bubbling period. Arelatively fast flushing of (say argon) inert gas is thereforepreferably used to insure substantially complete removal of the hydrogengas as a relatively low, safe concentration of hydrogen in the outflowgas.

Example II: Potassium A similar potassium, boron and tin tertiary alloymay be formed by utilizing an analogous technique, using a somewhatsmaller amount (by weight) of potassium borohydride. The followingproportion of original ingredients (again normalized to a total ofonegram) may be used:

Specific Approximate Example useful range Potassium borohydride 0.080 0.050-0. 140 Tin O. 920 0. 950-0. 860

(about cc. at 500 C.. (773 K)), or about 210 cc. at 600 C. (873K).Except for the somewhat different scraping procedure of the slag and thesomewhat lower inert gas flushing rate useable, the potassiumborohydride and, say, tin are processed in the same way to form thetertiary (potassium. boron and tin) alloy and the final coating.

Example III: Sodium-Potassium Mixed Alloy A sodium, potassium, boronand, say, tin 4-elemem alloy may be made in an analogous manner to thosepreviously described, for use in a spectral radiation lamp to obtainradiation in the characteristic spectral lines of both sodium andpotassium. Such a mixed alkali metal 4-element alloy may be made fromthe following proportions of starting ingredients (again normalized to aone gram total of reactants):

Specific Approximate The alloying technique is again essentiallyidentical to that of the general description in Example I with thefollowing minor differences. The total amount of evolved hydrogen (andtherefore the minimum sufficient inert gas flushing rate) will beintermediate between those of Examples 1 and II. Similarly although theslag formation is somewhat different in form, removal of these boroncompounds is essentially no more difficult than in Examples I and II.

As previously mentioned, during the initial alloying process (for all ofthe above examples and all analogous alloy formations), the temperatureshould be carefully raised slightly after the apparent completion of thedecomposition (and alloying) of the borohydride (i.e., after bubbling ofhydrogen ceases), to insure actual complete consumption of the alkalimetal borohydride(s), On the other hand, the temperature should never beraised much above that necessary to cause the particular result desired(e.g., melting of the additional metal", say, tin; decomposition of theborohydride at moderate rate; and melting of the final alloy duringfinal casting); such moderation in temperature lessens the possiblelosses of the alkali metal (as vapor) during the various manufacturingstages.

ALTERNATIVES AND CONCLUSION Although all of the specific examples givenabove utilize tin as the additional metal, other metals may be usedinstead. The additional metal should have reasonably satisfactoryphysical characteristics, the ability to alloy with the alkali metal inthe presence of boron, and be free of any spectroscopic interferencewith the alkali metal(s) spectral line emission. An example of a metalhaving such properties, which may therefore be substituted for the tinin the above examples, is lead (which has in fact been successfullytried).

In both examples I and II the alkali metals (sodium and potassiumrespectively) are initially introduced so as to form approximately 4-] 1percent (say 6 percent) by weight of the initial ingredients. In eachcase however a measurable but relatively small amount of the alkalimetal is lost in the form of the boron compounds in the slag and perhapseven as lost metal during the alloying process. In general the amount ofthe alkali metal (i.e., the sodium of Example I, the potassium ofExample II and the total of sodium and potassium in Example III) will bereduced from, say, about 6 percent to approximately 2 percent to 4percent by weight in the final alloy.

Similarly a substantial proportion of the boron is lost (primarily inthe slag material) so that its original proportion is probably halvedduring the manufacturing process. The residual boron, although typicallypresent as only a fractional percentage (by weight) in the total finalalloy, nevertheless has an appreciable effect in raising the meltingtemperature of the alloy (relative to a similar but boron-free alloy)and moderating the vapor pressure (at, for example. 350 C.) of thealkali metal.

The invention therefore provides a relatively simple technique forproviding an alkali metal alloy for a hollow cathode having desirablecharacteristics (and additionally provides a somewhat improved alloy forthis purpose having a somewhat higher melting point and lower vaporpressure than the most closely related previously used alloys. i.e., thebinary alloys of the alkali metals without any boron content. Theinventive process entirely avoids the handling of the pure alkalimetal(s) and the attendant problems and hazards, as well as requiringlittle equipment. It therefore is particularly suitable for formingrelatively small quantities of the alloy, for use, for example, inmaking only one or a few spectral radiation lamps at a time. Thehydrogen (and perhaps the boron as well) released during alloy formationreduces the oxides of the additional metal (often present on itssurface); in any event elimination of existing oxide from the finalalloy has actually been observed when either tin and lead (having somesurface oxidation) has been the additional" metal used.

Although three specific examples involving two alkali metals and amixture thereof have been specifically described, it will be obvious tothose skilled in the art that other alkali metals and mixtures thereofmay be used to form the analogous alloys. Similarly other additionalmetals (having the requisite moderately low melting points and otherdesirable alloying properties, as well as exhibiting no spectroscopicinterference with the alkali metal emission) may be utilized besides tinand lead. Further although specific proportions have been given in theExamples, obviously the relative proportions may be varied over arelatively large range. Because of these and other obviously possiblevariations, the invention is not limited to any of the details of anyone or more of the exemplary embodiments specifically disclosed; on thecontrary the invention is defined solely by the scope of the appendedclaims.

We claim:

1. In an improved cathode assembly for use in a radiation source lamp, acathode holder and a spectrally emitting coating on said cathode holder,said coating being an alloy comprising:

at least one alkali metal selected from the group consisting of sodium,potassium and mixtures thereof in the range of 2 percent to 10 percentby weight of the total constituents of said alloy;

boron in the range of A percent to -4 percent by weight of said totalalloy constituents; an additional metal selected from the groupconsisting of tin and lead in the range of 86 percent to 98 percent byweight of said total constituents,

said coating thereby being a tertiary alloy having a melting point notonly substantially above that of said one alkali metal, namely, sodium,potassium, or the mixture thereof, and of said additional metal, butalso somewhat above that of the corresponding, boron-free, binary alloyof said alkali metal and said additional metal; and

whereby an improved cathode assembly, capable of use with higheroperating lamp current and therefore capable of production of greaterintensity spectral radiation, is obtained.

2. An improved cathode assembly according to claim 1, in which:

said cathode holder is a generally cup-shaped hollow cathode, and

said alloy coating covers a substantial part of the interior surface ofthe hollow" thereof.

3. An improved cathode assembly according to claim 1, in

which; said additional metal comprises tin.

4. An improved cathode assembly according to claim 3, in which; saidcathode holder comprises titanium.

5. An improved cathode assembly according to claim 1, in which; saidadditional metal comprises lead.

2. An improved cathode assembly according to claim 1, in which: saidcathode holder is a generally cup-shaped hollow cathode, and said alloycoating covers a substantial part of the interior surface of the''''hollow'''' thereof.
 3. An improved cathode assembly according toclaim 1, in which; said additional metal comprises tin.
 4. An improvedcathode assembly according to claim 3, in which; said cathode holdercomprises titanium.
 5. An improved cathode assembly according to claim1, in which; said additional metal comprises lead.